At present, most of the steel rails widely used for railroads in China and foreign countries are made of eutectoid steel, the microscopic structure of which mainly consists of pearlites and contains trace ferrites, characterized in good toughness matching and moderate performance, etc. In the process of operation, under the complex stresses from the wheels and the friction force, the rail profile will change as the surface layer of rail head is worn down, causing degraded smoothness of train operation; especially, at sharp radius curves of heavy-load mixed passenger/freight transportation railroads and heavy-duty railroads, the inner steel rail may be crushed easily, and the outer steel rail may have severe side wearing; consequently, the condition of contact between the train wheels and the steel rails become severe and the dynamic interaction between the train wheels and the steel rails is increased, causing degraded operation quality, shortened service life of the steel rails, and increased maintenance cost of the railroad. The abrasion of the outer steel rail to the limit is one of the main causes for replacement of the steel rails at sharp radius curves.
At present, usually heat-treated steel rails with higher hardness rating are used at small radius curves of heavy-duty or heavy-load railroads, in order to improve the wear resistance of the steel rails by means of increasing hardness; in addition, other measures, such as applying oil to the steel rails and improving the contact condition between the train wheels and the steel rails, are taken at railroad sections where the wearing condition is harsh, in order to decrease the wearing rate. Compared with hot-rolled steel rails, heat-treated steel rails may have strength and hardness higher by 30% than those of hot-rolled steel rails, thanks to the strong refined crystalline strengthening effect produced in an accelerated cooling process, and may have toughness and plasticity equivalent or even superior to those of hot-rolled steel rails, which is to say, heat-treated steel rails have better overall strength and toughness properties. However, since heat-treated steel rails are usually cooled with compressed air, water-vapor mixture, or liquid water as the accelerated cooling medium, the effective cooling rate will be decreased quickly as the depth from the surface layer of rail head increases in the heat treatment process, resulting in significant difference in hardness between the deep zone in the rail head and the surface layer of rail head. For example, the hardness at 15 mm depth below the rail head surface can be lower by 2-3 HRC than that of the surface layer. In the actual service process of a steel rail, as the surface layer of rail head is worn down and the hardness is decreased, the wear resistance is decreased accordingly, and the service life of the steel rail exhibits a quick downtrend, adverse to the service life of the steel rail. Therefore, there is an urgent need for a heat treatment method for increasing the hardness within a 25 mm deep zone below the surface layer of rail head to a value comparable to the hardness of the surface layer, so as to obtain a deep-hardened pearlite steel rail and ensure persistent high service performance of the steel rail.